It is well documented that the current methods of long term storage of hazardous wastes are not acceptable. Burial at sea or in the earth presents the risk of wastes being dissolved if the containers housing them are damaged resulting in the entry of the wastes into the water table. It is necessary to store wastes in such a form that they cannot reenter the environment. It has been established that the most desirable waste form is a solid that can be easily transported and handled. Molecular traps of the present invention represent one possible solution to the problem of hazardous waste containment.
Current wasteform storage methods for low level radioactive wastes include storing wastes in tanks and burying them in the earth. This method relies on the stability of the storage vessel and a clay barrier either lining the pit or as packing material to keep wastes from contaminating the surrounding area. Once the storage vessel erodes or is damaged, water can enter and contact the waste. The clay barrier may be some protection but the waste will eventually diffuse around the clay particles or through cracks. U.S. Pat. No. 4,859,367 teaches of mixing waste with a geopolymer binder which is solidified. This essentially means the waste is encased in cement. Leaching experiments have determined that this is not a satisfactory method.
Ion-exchangers are another important class of materials used for the stabilization of waste. As taught in U.S. Pat. Nos. 3,167,504 and 4,272,382, ionic wastes can be selectively bonded to the ion-exchange material. There are many examples of different ion-exchangers used for the purification of waste streams but no attempt is made to seal the waste within the ion-exchanger after it is sorbed. Therefore the waste could be displaced if the material came in contact with more strongly bound ions and the toxins could be released into the environment.
U.S. Pat. No. 4,778,628 teaches using ion-exchangers as an added layer of barrier when storing chemical wastes. The wastes are not initially bound to the ion-exchanger but stored in sealed containers with a layer of ion-exchanger surrounding the containers. It is envisioned that if leakage occurs the wastes will have to go through a layer of ion-exchanger and could be retained. The disadvantage in this approach is that the wastes would not be permanently bound if a more strongly binding ion comes in contact with the material. The present invention is superior because by sealing of the waste within the structure of the molecular trap the possibility of release into the environment is eliminated.
Others have proposed the idea of structural containment that are radically different from the molecular trap process described herein. For example, it is taught that wastes can be first sorbed onto ion-exchange sites or porous materials such as glasses which are then sintered to collapse their pores, enclosing the wastes within the glass. For example, see U.S. Pat. Nos. 4,737,316, 4,780,239, 3,147,255 and 4,469,628. As this method involves restructuring solids, temperatures in the order of 1000.degree. C. are required and many wastes cannot be considered as they would volatilize at these temperatures. In addition, at these temperatures unwanted side reactions can take place. For example, the waste could react with components of the ion exchanger to form new phases which may segregate from the bulk and be more readily dissolved. The waste could also diffuse to grain boundaries or defect sites which can be more prone to leaching. As the current invention utilizes a unique low temperature rearrangement of the molecular sieve the need for high temperatures is eliminated. This makes it more economical and eliminates the possibility of unwanted side reactions. In addition, a wider variety of wastes can be considered for containment.
Another technology currently under development is called synroc, where hazardous wastes are mixed with a powder containing oxides of calcium, aluminum, zirconium, barium and titanium, calcined to dryness under a "reducing" atmosphere and then heated to high temperatures to form a ceramic. The hazardous waste (heavy metal or radionucleotide) becomes incorporated into the structure of the ceramic. Although this technology incorporates the wastes into the structural matrix of a material, as does the present invention, the mechanisms are drastically different. High temperatures are required to form the synroc material (1000.degree. C.). The present invention involves materials which are easier to synthesize, have a transformation that occurs under mild conditions and results in very stable oxide materials. In addition, the constituents of the present invention are inexpensive and once again a wider range of hazardous materials can be considered for containment.
A related idea involves synthesizing a ceramic material or zeolite in the presence of waste materials so that the waste becomes encased in the cavities of the zeolite and is therefore both structurally and chemically held. For example, see U.S. Pat. No. 3,959,172. This process will not work for a wide range of wastes because zeolite synthesis conditions are rather limited and the presence of a foreign species in solution can inhibit the desired structure from being produced. The present invention is much less limited as the desired channel system is synthesized independently and therefore reproducibly before the waste is introduced into it. As the transformation appears to be unaffected by the presence of sorbates the range of materials that can be encased is very broad.